As the name implies, a stent-graft is the combination of a flexible, compressible conduit, or graft, and a structural framework, or stent. The graft is a continuous sealed walled conduit surrounding a central plenum opened at opposing ends for the transport of blood or other body fluids. The graft is used to bridge diseased or weakened portions of vessels, or other corporeal lumen.
Problems with the Current Supported Grafts
The structural functions of a stent and graft are difficult to incorporate into a single integral unit, because each has a different function and each requires different material properties. Hence, the use of two distinct components. The connection of one component to another has been problematic. In most applications, the stent-graft is introduced in a collapsed state into the vessel. After the stent-graft is positioned in the vessel it is then expanded within the target lumen area of the vessel. The change in diameter may cause separation of the one from the other. The current art has many examples of ways to bind the stent to the graft, including sewing the stent and graft together, sandwiching the stent between an inner and outer grafts, or sandwiching the graft between inner and outer stents. But none achieve full integration. The resulting problems from the lack of full integration include micro-movement, poor contact with surrounding lumen, and restriction of the lumen, and the like.
Micromovement between the stent and graft can lead to graft erosion. Lack of contact between the stent and graft can lead to perigraft leakage, i.e. leakage of body fluids around the outer wall of the graft, especially when the stent is on the outside of the graft. Conversely, when the stent is on the inside of the graft, i.e. in the graft lumen, it can snag catheters and guidewires during re-instrumentation. The size of the stent-graft profile and the delivery system for the stent-graft can increase with the additional layers of stent or graft used in the sandwich method of attachment. In addition redundant folds of graft which is a problem with dual layer sandwich graft-stents, can fold into the lumen, occluding or impeding blood flow or other bodily fluid flow.
The Biology of Stent Ingrowth
Once implanted in a vascular lumen, most stents become rapidly incorporated into the wall of the surrounding vessel by smooth muscle invasion, followed by collagen deposition and intimal migration. The process of ingrowth varies according to the type of stent and the type of artery. Balloon-expanded stents incorporate more rapidly than self-expanding stents, and healthy canine arteries infiltrate the stent structure more rapidly than aging human arteries. It is believed that age is or would be a factor in the ingrowth of a stent in any mammal. The ingrowth of skeletal structures or frames for grafts in other biological vessels, such as the urethra or esophagus, is not well under stood as this point.